1. Field of the Invention
This invention relates to the apparatus for making corrugated or ribbed thermoplastic tubing of helical or annular form, the apparatus being of the type in which tubing of the thermoplastic material is continuously extruded into a tubular mold tunnel.
2. Background of the Invention
The tubular mold tunnel is defined by cooperatively interengaged mold blocks of a pair of mold assemblies, the mold blocks of each mold assembly being driven along a forward run in which the blocks define the modular block tunnel, and back along a return run. The mold blocks have molding surfaces comprising troughs for molding ribs of the tubing, and crests for molding grooves of the tubing. Such an apparatus is disclosed for example U.S. Pat. No. 4,319,872 issued on Mar. 16, 1982 to Gerd P. H. Lupke and Manfred A. A. Lupke.
With such known apparatus, it is desirable to apply vacuum between the mold blocks to encourage proper location of thermoplastic material against the mold face.
Each mold block may be a two part assembly of a carrier block which extends transversely between endless conveyor chains and a mold which is interchangeable on the carrier block for molding different sizes and configurations of tube.
When suction is to be applied to the mold face it may be drawn from stationary suction chambers above the mold block assemblies in the upper run and/or below the mold block assemblies in the lower run.
The carrier blocks carried by the endless conveyor chains may approach the suction chamber at the beginning of the mold tunnel. They are usually slidably engaged on the vacuum tunnel while forming part of the mold tunnel and receive suction from the suction chamber for transmission to the mold face. The carrier blocks may disengage from the suction chamber at any suitable point. For example the suction chamber may be elongate to run contiguously with the mold tunnel and the carrier blocks may disengage at the end of the suction chamber for their return run to reform the mold tunnel. Such mold blocks for applying suction are for example described in U.S. Pat. No. 4,319,872. However each mold block disclosed in that patent comprise a single block rather than the assembled mold and carrier block. Moreover, the suction chambers are arranged on the sides of the molds rather than along a bottom surface of the mold tunnel. Due to the fact that the carrier blocks must, as they are carried on the endless conveyor chains, readily engage the suction chamber smoothly and without interruptions, the tolerances between carrier block channels at suction chamber tracks have been quite large. Moreover, for greater smoothness and easy engagement the mating channels and track have tended to have rounded edges. While these precautions have normally led to smooth operation of the conveyor, sealing of the vacuum connection between the carrier block and the suction chamber has presented problems.